Process for the production of oleum and sulfuric acid

ABSTRACT

In the continuous production of oleum having a concentration of 10 to 45% by weight SO 3  and/or sulfuric acid having a concentration of 94 to 100% by weight H 2  SO 4  by burning sulfur with atomospheric oxygen on the principle of overstoichiometric or understoichiometric burning, cooling the resulting SO 2  -containing gas to 390°-480° C., catalytically reacting the cooled gas to SO 3  -containing gas on a vanadium-containing catalyst on the principle of single- or double-contact catalysis, absorbing the SO 3  -containing gas after cooling and, optionally, separating liquid from the gas after absorption, followed by recovering energy, the improvement which comprises effecting the burning of the sulfur with atomospheric oxygen in the presence of a dry SO 2  -containing gas which contains up to 5,000 ppm (NO) x  expressed as NO.

BACKGROUND OF THE INVENTION

This invention relates to a process for the production of oleum having aconcentration of 10 to 45% by weight SO₃ and/or sulfuric acid having aconcentration of 96 to 100% by weight H₂ SO₄ by burning of sulfur withatmospheric oxygen, reaction of the SO₂ -containing gases to SO₃-containing gases on vanadium catalysts and absorption of the SO₃-containing gases for the production of oleum and/or sulfuric acid.

Various processes for the production of oleum and sulfuric acid areknown from, and described in detail in, the literature.

In the thermal processes (burning of sulfur with atmospheric oxygen,roasting of pyrites, cleavage of sulfuric acid), nitrogen oxides(NO)_(x) are formed in addition to gases containing sulfur dioxide.These nitrogen oxides are absorbed in the same way as the gasescontaining sulfur trioxide which are formed from the SO₂ -containinggases on the vanadium catalyst, so that the sulfuric acid formed or theoleum is contaminated with absorbed (NO)_(x). For many applications,however, the sulfuric acid or the oleum should be free from absorbed(NO)_(x). In addition, the (NO)_(x) absorbed in oleum or sulfuric acidleads to increased corrosion of steel parts in the sulfuric acid plants.Accordingly, there has been no shortage of attempts to remove theimpurities from the sulfuric acid or oleum.

Thus, sulfuric acid free from nitrous vitriol and having a concentrationof >80% by weight can be produced by addition of hydrazine compounds at50 ° to 100° C. (DE-OS 1 792 348). Although the process of addingaqueous hydrazine sulfate solutions is basically suitable, it has beenfound that secondary reactions involving hydrazine sulfate take place atrelatively high contents of absorbed (NO)_(x) and merely create a falseimpression of a reduction in the (NO)_(x) content. In addition, thisprocess is very expensive.

In addition, it is known that monomeric or polymeric compoundscontaining NH₂ groups are added to remove nitrous acid from wastesulfuric acids (DE-OS 2 831 941). Hydroxylammonium sulfate, urea oramidosulfonic acid are preferably used as suitable compounds. However,this process can only be carried out for sulfuric acids having aconcentration of approximately 50 to 90% by weight. In addition, the useof the particularly active amidosulfonic acid, for example, involveshigh costs. Moreover, considerable effort is involved in the twoabove-mentioned processes in analyzing the sulfuric acid to monitor itsnitrogen content in order to avoid under- or overdosage of the compoundsadded.

Accordingly, the problem addressed by the present invention was toreduce the content of nitrous gases formed during the burning of sulfurfor the production of oleum and/or sulfuric acid and absorbed in oleumand/or sulfuric acid.

This problem has now been solved by the process according to theinvention.

SUMMARY OF THE INVENTION

The present invention relates to a process for the continuous productionof oleum having a concentration of 10 to 45% by weight SO₃ and/orsulfuric acid having a concentration of 94 to 100% by weight H₂ SO₄ byburning of sulfur with atmospheric oxygen on the principle ofoverstoichiometric or understoichiometric burning, cooling of theresulting SO₂ -containing gases to 390°-480° C., catalytic reaction ofthe cooled gases to SO₃ -containing gases on a vanadium-containingcatalyst on the principle of single- or double-contact catalysis,absorption of the SO₃ -containing gases after cooling and, optionally,separation of liquids from the gases after absorption, followed byenergy recovery, characterized in that the burning of sulfur withatmospheric oxygen is carried out with addition of dry SO₂ -containinggases which contain up to 5,000 ppm (NO)_(x) and preferably less than2,000 ppm (NO)_(x), expressed as NO.

The dry SO₂ -containing gases normally contain more than 50 ppm(NO)_(x), expressed as NO.

Dry cleavage gases from the thermal cleavage of waste sulfuric acids arepreferably used as the SO₂ -containing dry gases which may contain up to5,000 ppm (NO)_(x), expressed as NO. These cleavage gases preferablycontain 5 to 10% by volume O₂, 5 to 8% by volume SO₂,<200 ppm CO, <1,000ppm (NO)_(x), <50 ppm hydrocarbon compounds and 82 to 90% by volume H₂O, N₂ and CO₂.

In another variant of the process, combustion gases from the burning ofsulfur-containing materials are used as the SO₂ -containing dry gaseswhich may contain up to 5,000 ppm (NO)_(x).

The ratio between the quantity of SO₂ from the dry SO₂ -containing gasesadded and the quantity of SO₂ formed from the burning of sulfur ispreferably from 1:5 and 3:1.

To carry out the process in accordance with the invention, the dry SO₂-containing gases which may contain up to 5,000 ppm (NO)_(x) may bedirectly introduced into the furnace chamber in which sulfur is burntwith oxygen or may be introduced into the furnace chamber after mixingwith combustion air. It is important that the SO₂ -containing gaseswhich may contain up to 5,000 ppm (NO)_(x) (nitrous-vitriol-containingSO₂ -containing gases) are burnt together with the sulfur. Up to 95% byweight of the total nitrogen oxides can be removed by the processaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one preferred embodiment of the process, the sulfur is burnt attemperatures in the range from 500° C. to 1,000° C. (as measured at theexit of the furnace chamber before cooling of the gases) and preferablyat temperatures in the range from 700° C. to 950° C.

The SO₃ -containing gases are preferably absorbed in oleum having aconcentration of 15 to 45% by weight SO₃ and preferably 20 to 30% byweight SO₃.

The energy released during the burning of sulfur may be recovered in theusual way with formation of wet steam at different pressure stages.

The liquids removed from the gases after absorption may be recycled tosulfuric acid processing plants, preferably sulfuric acid cleavageplants.

The catalysis conditions in the practical application of the process arenot critical. The known methods of regulating concentration may be usedfor the absorption of SO₃ to oleum.

BRIEF DESCRIPTION OF THE DRAWING

The figure is a flow sheet of one possible embodiment of the processaccording to the invention although the process according to theinvention is by no means limited to this embodiment. The referencenumerals identify the following elements:

    ______________________________________                                         1       air dry (drying tower)                                                2       gas dryer (drying tower                                               3       fan (blower)                                                          4       heat exchanger                                                        5       sulfur burner                                                         6       heat exchanger                                                        7       primary catalyst (pre-contact 3 bed converter)                        8       heat exchanger                                                        9       waste heat exchanger                                                 10       oleum absorber                                                       11       intermediate absorber                                                12       filter (mist eliminator)                                             13       secondary catalyst (post-contact)                                    14       final absorber                                                       15       filter (mist eliminator)                                             16       liquid                                                               17       air                                                                  18       SO.sub.2 -containing gas                                             19       sulfur                                                               20       oleum                                                                21       sulfuric acid                                                        22       flue gas. (off-gas)                                                  ______________________________________                                    

The following examples are intended to illustrate the process accordingto the invention. without limiting it in any way.

EXAMPLE Example 1

In an industrial plant for the production of oleum having aconcentration of 15 to 38% by weight free SO₃ and sulfuric acid having aconcentration of 95.5 to 99.0% by weight H₂ SO₄, which is based on theburning of elemental sulfur on the principle of the overstoichiometricburning of sulfur, the double contact process and double absorption,18,600 Nm³ /h SO₂ -containing gas having a temperature of 62° C. and acontent of 6.3% by volume SO₂, 5.3% by volume O₂ and 200 mg NO/Nm³(remainder CO₂ and N₂), which has been dried with 96.7% by weightsulfuric acid in a gas dryer, are mixed with air-dried at 42° C. in anair dryer--to form a total gas volume of 46,000 Nm³ /h. This mixture istaken in under suction by a fan. The SO₂ -containing gas mixture isheated to 263° C. in a heat exchanger utilizing the heat from the firstand second fixed bed trays is mixed and burnt with 4.42 t/h finelydivided liquid sulfur in a sulfur burner. The liquid sulfur is dispersedvia a nozzle stock surmounted by a nozzle. The hot gas (920° C.) iscooled to 435° C. and fed to a first catalyst consisting of threefixed-bed trays. The gas entering fixed bed tray 1 has the followingcomposition: 8.5% by volume SO₂, 9.5% by volume O₂, 38 mg NO/Nm³(remainder CO₂ and N₂), corresponding to an NO reduction of 53%.

After leaving the 3rd fixed bed tray with a temperature of 512° C., theSO₃ - and SO₂ -containing gas is cooled to 215° C. in another heatexchanger and in a waste heat exchanger. A partial gas stream ofapproximately 50% is fed to the oleum absorption stage. The compositionof the circulating oleum (62° C.) was 20.7% by weight free SO₃, 12 ppm N(as determined by the Devarda method) and 28 ppm N (as determined by theKjeldahl method). The remaining partial gas stream is fed together withthe gas stream from the oleum absorption stage to the intermediateabsorber in which a sulfuric acid having a concentration of 98.95% byweight H₂ SO₄ and a temperature of 82° C. is present. The remaining SO₃is completely absorbed there. The remaining SO₂ -containing gas isbrought to a temperature of 440° C. in heat exchangers and oxidized onthe secondary catalyst in another fixed bed tray. After reaction of theremaining SO₂, the gas leaves the tray at 420° C., is cooled in the heatexchanger and introduced into the final absorber (containing 99.9% byweight H₂ SO₄ at a temperature of 87° C). After absorption of the SO₃,the gas is cleaned in a Brinkmist filter and emitted with an SO₂ contentof 146 ppm. In the absorbers, the nitrite value is <1 mg/kg oleum orsulfuric acid. The oleum accumulating is fed to the intermediateabsorber. The total production of approx. 358 tons SO₃ per day isdischarged as 98.95% by weight H₂ SO₄. The liquid of H₂ SO₄ containingapprox. 2.5% nitrosylsulfuric acid which accumulates in the waste heatexchanger in a quantity of approx. 9 1/24 h is delivered together withthe filter effluents to the-acid cleavage plant. 14 t/h 30 barsteam/300° C. are produced.

Example 2

As in Example 1, 2.6 t/h liquid sulfur were processed with 25,600 Nm³SO₂ -containing gas consisting of 6.4% by volume SO₂, 5.0% by volume O₂,200 mg NO/Nm₃ (rest N₂ and CO₂) and with air in a total gas quantity of47,000 Nm³ /h. The furnace temperature in the burner was 760° C. and thecomposition of the gas (temperature 446° C.) on entering tray 1 was 8.1%by volume SO₂, 7.1% by volume O₂ and 28 mg NO/Nm³ (rest N₂ and CO₂),which corresponds to an NO reduction of 74%, based on the NO content ofthe SO₂ -containing gas used (the NO₂ -content is approximately 1 to 2mg/Nm³). The absorbers had been filled with 26.7% by weight oleum,99.07% by weight H₂ SO₄ and 99.04% by weight H₂ SO₄. The emissioncomprised 130 ppm SO₂. 300 tons SO₃ were produced in the form of 99.07%by weight H₂ SO₄. The output of 30 bar steam was 8.5 t/h.

Example 3

As in Example 1, 4.2 t/h liquid sulfur were processed with 24,400 Nm³SO₂ -containing gas consisting of 6.9% by volume SO₂, 5.6% by volume O₂and 150 mg NO/Nm₃ (rest N₂ and CO₂) and with air in a total gas quantityof 47,000 Nm³ /h. The furnace temperature in the burner was 930° C. andthe composition of the gas (temperature 454° C.) on entering tray 1 ofthe primary catalytst was 9.2% by volume SO₂, 8.0% by volume O₂ and 27ppm NO/Nm³, which corresponds to an NO degradation of 65%, based on theNO content of the SO₂ -containing gas used. The absorbers had beenfilled with 24.70% by weight oleum containing 7 ppm N (as determined bythe Devarda method) and 19 ppm (as determined by the Kjeldahl method) ,99.05% by weight H₂ SO₄ and 99.10 % by weight H₂ SO₄. The emissioncomprised 230 ppm SO₂. 400 t SO₃ /24 h were produced in the form of99.10% by weight H₂ SO₄. The output of 30 bar steam was 14 t/h.

Example 4 Production of SO₂ -containing gases from the cleavage ofsulfuric acid

68% by weight waste sulfuric acid, salt content 0.5% by weight, was fedinto a lined furnace heated with fuel oil S (1.8% by weight sulfur). Thefurnace temperature was 1,050° C. The throughputs per unit of time wereas follows: air 12,000 m³, fuel oil 1,000 kg, cleavage acid 7,000 kg. Atthe same time, a waste gas from an intermediate organic productionprocess was introduced in a quantity of 50 m³ per unit of time: SO₂ 45%by volume, hydrocarbon compounds 2.5% by volume, N₂ and O₂ 52.5% byvolume. The efficiency of the H₂ SO₄ →H₂ +SO₂ +1/2O₂ reaction is 98% ofthe theoretical. The gas leaving the cleavage furnace is cooled to 800°C. in a recuperator, the burner air used for cooling being heated to420° C. In a wet quench, the hot cleavage gas is cooled to 80° C. byintroduction of an excess of water. The water deposited is removed by astripper. The steam-saturated gas with a temperature of approximately80° C. enters a packed washing tower which may be operated in co-currentor countercurrent. A 2% by weight sulfuric acid is used as the washingliquid. The sensible.heat is dissipated by external, indirect coolingwith water. Liquids deposited are removed from the system by a stripper.The gas mixture (temperature 35° C.) leaving the washing system, whichhas the following composition: O₂ 6.5% by volume, SO₂ 6.0% by volume,NO_(x) 125 ppm, CH compounds<1 ppm, H₂ O, CO₂ and N₂ 87.5% by volume, isafter-cleaned (wet EGC) and dried with 97.5% by weight sulfuric acid.

It will be appreciated that the instant specification is set forth byway of illustration and not limitation, and that various modificationsand changes may be made without departing from the spirit and scope ofthe present invention.

What is claimed is:
 1. In the continuous production of oleum having aconcentration of 10 to 45% by weight SO₃ or sulfuric acid having aconcentration of 94 to 100% by weight H₂ SO₄ by burning sulfur withatmospheric oxygen, cooling the resulting SO₂ -containing gas to390°-480° C., catalytically reacting the cooled gas to SO₃ -containinggas on a vanadium-containing catalyst, absorbing the SO₃ -containing gasafter cooling and separating liquid from the gas after absorption,followed by recovering energy, the improvement which comprises effectingthe burning of the sulfur with atmospheric oxygen in the presence of adry SO₂ -containing gas which contains from about 50 ppm up to about5,000 ppm (NO)_(x) expressed as NO.
 2. A process according to claim 1,wherein the dry SO₂ -containing gas which contains up to 5000 ppm(NO)_(x) expressed as NO is dry cleavage gas from the thermal cleavageof waste sulfuric acid.
 3. A process according to claim 2, wherein thedry cleavage gas contains 5 to 10% by volume O₂, 5 to 8% by volume SO₂,<200 ppm CO, <1,000 ppm (NO)_(x), 50 ppm hydrocarbon compounds and 82 to90% by volume H₂ O, N₂ and CO₂.
 4. A process according to claim 1,wherein the dry SO₂ -containing gas which contains up to 5,000 ppm(NO)_(x) comprises combustion gas from the burning of asulfur-containing material.
 5. A process according to claim 1, whereinthe ratio between the quantity from the dry SO₂ -containing gas and thequantity of SO₂ from the burning of sulfur is from 1:5 to 3:1.
 6. Aprocess according to claim 1, wherein the dry SO₂ -containing gas whichcontains up to 5,000 ppm (NO)_(x) is directly introduced into thefurnace chamber where the sulfur is burnt.
 7. A process according toclaim 1, wherein the dry SO₂ -containing gas is mixed with theatomospheric oxygen and the gas mixture is introduced into the-furnacechamber where the sulfur is burnt.
 8. A process according to claim 1,wherein the sulfur is burnt at a temperature in the range from 500° C.to 1,000° C., as measured at the exit of the furnace chamber beforecooling of the gases.
 9. A process according to claim 1, wherein the SO₃-containing gases are absorbed in oleum having a concentration of 15 to40% by weight SO₃.
 10. A process according to claim 1, wherein liquid isseparated off frcm the gas after absorptien and is recycled to asulfuric acid processing plant.
 11. A process according to claim 1,wherein the burning of the sulfur is effected in the presence of a drySO₂ -containing gas which contains less than 2,000 ppm (NO)_(x)expressed as NO.
 12. A process according to claim 1, wherein the sulfuris burnt at a temperature in the range from 700° C. to 950° C., asmeasured at the exit of the furnace chamber before cooling of the gases.13. A process according to claim 1, wherein the SO₃ -containing gaselsare absorbed in oleum having a concentration of 20 to 30% by weight SO₃.14. A process according to claim 1, wherein liquid is separated off fromthe gas after absorption and is recycled to a sulfuric acid cleavageplant.